The present invention relates to phase-locked loop frequency synthesizers and, more particularly, to such a frequency synthesizer not having the uncontrolled small frequency modulation of its output typical of the prior art.
The makers of electronic equipment are finding phase-locked loop frequency synthesizers to be important components of many items. For example, synthesizers of this type having voltage controlled oscillators are now becoming widely used as digital tuners for communication transmitters and receivers. They are also now commonly used in controllers for variable speed motors.
Generally, a phase-locked loop frequency synthesizer relies for control of the frequency of its output signal on a comparison of the frequency of an oscillatory signal derived from such output with the frequency of an oscillation generated by a reference oscillator. The comparisons are typically made by phase/frequency detectors, such as of the type included in the integrated circuit chip sold by the Fairchild Semiconductor Components Group of Fairchild Camera and Instrument Corporation with the component designation 11C44. The technology of phase/frequency detectors, however, leaves much to be desired. Because of internal timing delays, such detectors are incapable of detecting small differences in the phase between the reference oscillator signal and the derived comparison signal. The result is that the phase of the synthesizer output can drift to some degree without being corrected. When the degree of drift becomes sufficiently large to enable detection by the phase/frequency detector, the detector will react by generating a phase correction signal for application to the synthesizer output. Thus, because of this "dead band" in phase difference which cannot be detected by the phase/frequency detector, the frequency of the synthesizer output will oscillate about or, in other words, "hunt" around, the desired frequency. This spurious modulation of the frequency is especially undesirable in communications applications in which the output of the synthesizer is deliberately modulated with a communication information signal. Such spurious modulations appear as increased noise and distortion. Also, in controllers for variable speed motors, the spurious modulations cause speed control variations which consequently reduce the preciseness of the controller and limits the uses for which variable speed motors are appropriate.